Reverse osmosis is a process for purification of saline water. In accordance with this process, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the feed solution to separate purified water using a semipermeable membrane. Purified water diffuses through the membrane while salts and other impurities are retained by the membrane.
The mechanical properties of the membranes often require reinforcement or support, for example, with polyester woven or nonwoven fabric, in order to prepare satisfactory reverse osmosis structures. Typically, a reverse osmosis membrane is formed by coating or casting an appropriate polymer (e.g., polysulfone) solution onto the supporting fabric. With the coating of extremely thin membranes, continuity of the membrane on the supporting surface is difficult to obtain. Moreover, the lack of proper adhesion between the supporting fabric and the reverse osmosis membrane can lead to delamination in use, which results in the formation of blisters between the supporting fabric and the reverse osmosis membrane.
Fabrics used as a support substrate in reverse osmosis applications are preferably low in cost, stable under the operating conditions encountered in reverse osmosis operations and highly porous so as to retain high flux in operation.
A support material disclosed in U.S. Pat. No. 4,454,176 to Buckfelder is prepared by weaving a yarn of polyester. The woven cloth is dried, heat set and calendared, and used as the reverse osmosis membrane substrate. Other support fabrics used by Buckfelder to make exemplary supported reverse osmosis membranes include a calendared spunbonded polyester fabric and a resin-bonded polyester fabric.
U.S. Pat. No. 4,728,394 to Shinjou discloses a semipermeable membrane support of the type which is used for precise filtrations such as in ultrafiltration and reverse osmosis. Such precise filtration is utilized in desalination of seawater, the food industry, treatment of industrial wastewater and medical applications. The Shinjou '394 patent discloses that semipermeable membranes comprising synthetic polymer, without any support, are inferior in mechanical strength. Therefore such membranes are generally used with a backing material such as woven fabric. For example, reverse osmosis membranes are formed by casting a polymer solution directly onto a porous support such as a nonwoven fabric. Conventional membrane support substrates include woven and knitted fabrics, nonwoven fabric, porous sintered material, or paper.
In particular, various support substrates which use nonwoven fabric have been developed. However, supports having a high density cause insufficient penetration of the polymer solution. A consequence is the delamination between layers due to reduced peeling strength between the support and membrane. Another consequence can be the generation of pinholes due to residual bubbles, because of insufficient debubbling in the support. Low-density support substrates, in spite of sufficient penetration by the polymer solution, result in over penetration of the polymer solution to the back surface opposite to the casted surface. The resulting membranes give uneven filtration. Severe defects such as reduced filtration performance and/or damage of the semipermeable membrane due to the partial excessive pressurization during filtration operation can result.
The Shinjou '394 patent proposes to solve the foregoing problems by forming a low-density nonwoven layer to a high-density nonwoven layer and laminating the layers using a heated calendar. Then a polymer solution is cast on the low-density layer of the laminated support. The low-density nonwoven layer is formed by a dry process and the high-density nonwoven layer is formed by a wet process. All of the fibers in both layers are polyester. The dry-processed polyester nonwoven layer comprises 20-80%. preferably 30-60%, binder fibers, which encompasses both undrawn polyester fibers and conjugate (i.e., bicomponent) polyester fibers. The wet-processed nonwoven layer is formed entirely of polyester fibers having a denier of 1.5 or less, comprising 30-90%, preferably 40-70%, undrawn or conjugate polyester fibers. The wet process is a conventional papermaking process followed by heated calendaring. In the examples of Shinjou, the wet-processed nonwoven consisted of 50% polyester staple fibers having a denier of 1.0 and a length of 5 mm and 50% of undrawn polyester fibers having a denier of 1.0 and a length of 5 mm.
The foregoing prior art does not disclose a non-woven support substrate made by a wet process on a papermaking machine without lamination to a dry-processed nonwoven web.